Rivet analyses

// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {


  /// @brief Lambda_c -> Lambda e+ nu_e asymmetry
  class CLEOII_2005_I668268 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(CLEOII_2005_I668268);


    /// @name Analysis methods
    /// @{

    /// Book histograms and initialise projections before the run
    void init() {
      // Initialise and register projections
      declare(UnstableParticles(), "UFS");

      // Book histograms
      book(_h_Lambda, "/TMP/hLambda", 20, -1., 1.);
    }

    void findChildren(Particle parent,
                      int sign,
                      unsigned int& npart,
                      Particles& lambda,
                      Particles& e,
                      Particles& nu) {
      for (const Particle& child : parent.children()) {
        if (child.pid() == sign * PID::LAMBDA) {
          lambda.push_back(child);
          ++npart;
        }
        else if (child.pid() == -sign * PID::EMINUS) {
          e.push_back(child);
          ++npart;
        }
        else if (child.pid() == sign * PID::NU_E) {
          nu.push_back(child);
          ++npart;
        }
        else if (!child.children().empty()) {
          findChildren(child, sign, npart, lambda, e, nu);
        }
        else {
          ++npart;
        }
      }
    }

    /// Perform the per-event analysis
    void analyze(const Event& event) {
      // loop over Lambda_c baryons
      for (const Particle& Lambdac : apply<UnstableParticles>(event, "UFS").particles(Cuts::abspid == 4122)) {
        int sign = Lambdac.pid() / 4122;
        Particles lambda, e, nu;
        unsigned int npart(0);
        findChildren(Lambdac, sign, npart, lambda, e, nu);
        if (npart != 3 || lambda.size() != 1 || e.size() != 1 || nu.size() != 1) continue;
        Particle baryon2;
        if (lambda[0].children()[0].pid() == sign * 2212 && lambda[0].children()[1].pid() == -sign * 211) {
          baryon2 = lambda[0].children()[0];
        }
        else if (lambda[0].children()[1].pid() == sign * 2212
                 && lambda[0].children()[0].pid() == -sign * 211) {
          baryon2 = lambda[0].children()[1];
        }
        else
          continue;
        // first boost to the Lambdac rest frame
        LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(Lambdac.momentum().betaVec());
        FourMomentum pbaryon1 = boost1.transform(lambda[0].momentum());
        FourMomentum pbaryon2 = boost1.transform(baryon2.momentum());
        // to lambda rest frame
        LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(pbaryon1.betaVec());
        Vector3 axis = pbaryon1.p3().unit();
        FourMomentum pp = boost2.transform(pbaryon2);
        // calculate angle
        double cTheta = pp.p3().unit().dot(axis);
        _h_Lambda->fill(cTheta);
      }
    }

    pair<double, double> calcAlpha(Histo1DPtr hist) {
      if (hist->numEntries() == 0.) return make_pair(0., 0.);
      double sum1(0.), sum2(0.);
      for (const auto& bin : hist->bins()) {
        double Oi = bin.sumW();
        if (Oi == 0.) continue;
        double ai = 0.5 * (bin.xMax() - bin.xMin());
        double bi = 0.5 * ai * (bin.xMax() + bin.xMin());
        double Ei = bin.errW();
        sum1 += sqr(bi / Ei);
        sum2 += bi / sqr(Ei) * (Oi - ai);
      }
      return make_pair(sum2 / sum1, sqrt(1. / sum1));
    }

    /// Normalise histograms etc., after the run
    void finalize() {
      //  asymmetry
      normalize(_h_Lambda);
      Estimate0DPtr _h_alpha;
      book(_h_alpha, 1, 1, 1);
      pair<double, double> alpha = calcAlpha(_h_Lambda);
      _h_alpha->set(alpha.first, alpha.second);
    }
    /// @}


    /// @name Histograms
    /// @{
    Histo1DPtr _h_Lambda;
    /// @}
  };


  RIVET_DECLARE_PLUGIN(CLEOII_2005_I668268);

}